The present invention relates to a braking pressure control unit for a hydraulic brake system, in which the brakes at the wheels of one axle are acted upon by two separate brake circuits comprising a casing with an intermediate piston arranged therein, the intermediate piston being movable from its normal position in both directions relative to the casing, two differential pistons slidable in the casing against a preloading force in opposite directions, and two valve devices by which a pressure input chamber is connectible to a pressure output chamber, with one of the two differential pistons forming one part and the intermediate piston the other part of each of the two valve devices.
A braking pressure control unit of this type is known from British Pat. No. 1,475,635. In this arrangement, each control portion of the intermediate piston bears against a casing projection via a spring system of its own. Conditioned by the differences in pressure of the two brake circuits always occurring in a tandem master cylinder and the tolerances of the springs acting upon the control portions of the intermediate piston considerable discrepancies of the outlet pressures of the braking pressure control unit may be the consequence. Another disadvantage of the known device is that upon failure of a brake circuit, the control function of the other brake circuit is maintained unaffected. It is, however, desired when a brake circuit fails to obtain a maximum high braking effect with the still operable brake circuit.